Conventional electrical metering systems include a plurality of electrical meters, such as induction or electronic type watthour meters, for measuring the energy consumption of individual electrical energy users. Each electrical meter generally includes metering circuitry for measuring the power consumption of the electrical load associated with the meter and for tabulating the load data related to the measured power consumption. The metering circuitry of conventional electrical meters generally includes an internal current sensor mounted within the housing of the electrical meter for sensing the line current supplied to the associated electrical load and for producing an output signal related to the line current.
For example, conventional induction watthour meters include a current sensing circuit in which the conductor carrying the line current is wrapped about an iron core to form a current coil. Magnetic flux is created in the iron core due to the line current which, in combination with the magnetic flux from a similar voltage coil, rotates a disc at a rate related, according to a predetermined ratio, to the customer's electrical energy consumption rate to thereby measure the power consumption of the associated electrical load.
In a number of instances, the range of line currents is relatively large, from very small line currents to relatively large line currents, such as 800 A, for example. Thus, the size of the conductor about the iron core to measure the relatively large range of line currents and to produce an output appropriately scaled output becomes prohibitively large. In such instances, a current transformer is typically employed in conjunction with the electrical meter and the internal current sensing device of the meter. In particular, a current transformer is generally disposed between the transmission line carrying the line current and the current sensing circuit and is located external to the electrical meter. For example, in an electrical metering system supplying electrical power to a three-phase electrical load, current transformers are typically connected to two or more of the bus bars extending from the switch gear of the electrical metering system to the electrical load.
Conventional current transformers also generally include an iron core about which a conductor carrying the line current is wrapped to form a current coil. The magnetic flux created in the iron core can, in turn, create a scaled output current, proportional to the line current which is supplied to the electrical load. The output current is sensed and further scaled by the internal current sensor of the electrical meter and the power consumption of the associated electrical load is computed, in part, therefrom. Accordingly, a relatively large transformation ratio can be established between the line current and the output signal of the internal current sensor of the electrical meter, such as 400,000:1 to convert 800 A of line current to a scaled output current of 2 mA, for example, by the combination of a current transformer and the internal current sensing device of an electrical meter.
The need for large current transformation ratios is particularly apparent in electrical metering systems which include electronic electricity meters, such as electronic watthour meters or other electronic metering devices. Exemplary electronic electricity meters include GE Type EV.TM. and Phase3.TM. meters which are sold by General Electric Company of Somersworth, N.H. These electronic electricity meters typically include integrated circuits for measuring the current and voltage usage of individual electrical energy customers. The integrated circuits are generally Application Specific Integrated Circuits ("ASIC") which are designed to accept and measure small signal levels, such as less than 2 mA and less than 5 volts. The combination of the current transformers and the internal current sensors of such electronic electricity meters must thus provide a large transformation ratio to scale the relatively large line currents, such as 800 A, to the relatively small input levels, such as 2 mA, accepted by the integrated circuits. As described above, the number of windings required to properly scale the line current and to provide the required transformation ratio can quickly become prohibitively large and expensive.
Various other types of current sensors have been developed for incorporation into an electrical meter which scale a line current to a level measurable by an electrical meter. For example, the current sensors described in U.S. Pat. No. 4,182,982 to Wolf, et al. which issued Jan. 8, 1980 and U.S. Pat. No. 4,492,919 to Milkovic which issued Jan. 8, 1985 include one or more main shunt paths and a parallel, auxiliary shunt path between which the line current divides based upon their respective cross-sectional areas. The resulting current conducted by the auxiliary shunt path can be sensed to produce a scaled output signal.
In addition, a coaxial current sensor is disclosed in U.S. Pat. No. 5,066,904 to Bullock which issued on Nov. 19, 1991 and is assigned to the assignee of the present invention. The coaxial current sensor divides the line current between two coaxially-arranged conductors. The center conductor of the coaxial current sensor extends through the bore of a toroidal core and induces a magnetomotive force in the toroidal core from which a scaled output signal can be produced.
Still further, a differential current sensor is disclosed in U.S. patent application Ser. No. 08/043,903 to Coburn filed Apr. 7, 1993, now abandoned, and also assigned to the assignee of the present invention. The differential current sensor divides a line current into first and second portions according to a predetermined ratio and senses the current differential therebetween. Based upon the sensed current differential, a scaled output current can be produced.